In a conventional black-and-white image forming apparatus, such as a copying machine, a printing machine, and a facsimile machine, a latent image is formed on an image carrier, such as a photosensitive drum or a photosensitive belt, by a latent image forming unit. Such a latent image is developed by a developing unit to form a toner image, which is then transferred to a recording sheet and fixed by a fixing device.
In a color image forming apparatus, such as a color copying machine, a color printer, and a color facsimile machine, a plurality of latent images are formed on an image carrier, such as a photosensitive drum and a photosensitive belt. The latent images are developed with toners in different colors by a developing unit. The toner images in different colors are superimposed on each other to form a color image, which is then transferred to a material such as a recording sheet and fixed by a fixing device.
FIG. 6 shows a conventional thermal roller fixing device. In the thermal roller fixing device, a fixing roller 21 presses a pressure roller 22, and the fixing roller 21 is heated by a heater 23 disposed therein. A recording sheet as a material onto which a toner image is to be fixed is transported through the nip portion between the fixing roller 21 and the pressure roller 22. A toner image formed on the recording sheet is by the heat from the fixing roller 21 and the pressure from the pressure roller 22.
In the fixing device, oil is applied to the fixing roller to prevent offset. In a color image forming apparatus, it is necessary to use toners in different colors having a low viscosity to obtain a high-quality image having a high glossiness and a high light transmission rate. Therefore, oil is applied to the fixing roller to prevent offset.
Japanese Utility Model Application Laid-Open No. 62-60956 discloses a fixing device which has an endless-type fixing belt stretched over a fixing roller having a heat source therein and an idle roller having a diameter smaller than that of the fixing roller. The fixing belt has a high heat conductivity and a high separability, and is stretched over the fixing roller and the idle roller, with the separable surface facing outward. A pressure roller is brought into contact with the fixing belt in a point closer to the idle roller than the separation point of the fixing roller.
Japanese Patent Application Laid-Open No. 6-318001 discloses a fixing device which can produce clear fixed images without offset. The separability between the fixing belt and the toners is improved so that no or little oil is required. In this fixing device, an endless-type fixing belt having a release layer on the front surface of its belt main body is stretched between the surface-elastic fixing roller and the heating roller. A pressure roller presses the fixing roller from below via the fixing belt, thereby forming a nip portion between the fixing belt and the pressure roller. A recording medium support member disposed below the fixing belt between the heating roller and the nip portion forms a straight heating passage to the fixing belt. The heat capacity of the fixing belt is 0.002 to 0.025 cal/.degree. C. per 1 cm.sup.2.
In a fixing device disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 6-318001 or Japanese Patent Application Laid-Open No. 8-137306, a toner image is atmosphere-preheated. The fixing effects and offset restrictions can be strengthened in a low-speed operation, but they cannot be improved in a high-speed operation. Moreover, to obtain effects from the preheat, the distance between the fixing belt B and the recording medium support member G needs to be narrowed, resulting in a problem of scratches in a non-fixed toner image.
In a fixing device disclosed in the above-mentioned Japanese Utility Model Application Laid-Open No. 62-60956 or Japanese Patent Application Laid-Open No. 4-273279, a fixing process is carried out to fix a toner image on a recording sheet via a belt, followed by a transporting process to transport the recording sheet by the belt and a removing process to remove the fixed toner image from the belt. The transportation of the recording sheet after the fixing process is unstable, often causing unevenness in glossiness and hot offset. Since the transfer sheet and the toners after passing through the nip portion is further subjected to the remained heat of the belt, the toners unduly melt and often cause offset. Also, due to the unsteady transportation of recording sheets, the temperature on the image surface of a recording sheet becomes partially uneven, which causes unevenness in glossiness.
A thermal roller fixing device comprising a thin roller and a glass roller to shorten the rise time is well known. Such a fixing device is disclosed in Japanese Patent Application Laid-Open Nos. 53-89744, 59-65867, 4-77917, and others. To shorten the rise time of this thermal roller fixing device, the heater has a large capacity, while the fixing roller has a low heat capacity. Because of this, the fixing roller is made thin and small in diameter, or the glass roller is employed to directly heat the surface of the fixing roller in an instant by radiated heat.
In the above thermal roller fixing device, however, the thin fixing roller having a small diameter and a low heat capacity is not strong enough. The low fixing pressure and the narrow nip width result in insufficient fixing. The thermal roller fixing device cannot provide desired fixing effects in a high-speed machine.
In a fixing device of a color image forming apparatus, the toners of each of colors overlapped on a recording material are melted enough to be fixed to the recording material. A low fixing pressure often causes a fault in fixing. Therefore, it is necessary to keep the fixing pressure high. Since the fixing roller cannot be made thin and small in diameter to maintain its strength, it is difficult to employ a thin roller and a glass roller to shorten the rise time in the conventional thermal roller fixing device.
Also, to obtain a high-quality image having a high glossiness and a high light transmission rate in a color image forming apparatus, it is necessary to use toners in different colors having a low viscosity. Therefore, oil must be applied to the fixing roller to prevent offset.
In a fixing device disclosed in Japanese Patent Application Laid-Open No. 4-324476, two heaters are disposed on the side of the front surface of a recording sheet, and they are switched depending on the copying mode. In this case, the recording sheet receives heat only on its front surface, and offset is not efficiently prevented. In this fixing device, a separation claw is also necessary to separate a recording sheet from the fixing roller and the pressure roller. In a fixing device disclosed in Japanese Patent Application Laid-Open No. 62-23260, a curl removing roller is necessary to prevent a recording sheet from curling.